This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Wireless 5G (fifth generation) systems will not only be “4G (fourth generation), but faster”. One of novel features discussed in relation to 5G is Ultra-Reliable Communication (URC), an operation mode not present in today's wireless systems. URC refers to provision of a certain level of communication service almost 100% of the time. Example URC applications include reliable cloud connectivity, critical connections for industrial automation and reliable wireless coordination among vehicles. URC represents a new performance feature that needs to be targeted in designing future wireless systems. The URC in 5G is expected to place stringent requirements on connection reliability and latency. In an extreme case, the reliability greater than 99.999% might be required (see Mobile and wireless communications Enablers for the Twenty-twenty Information Society (METIS), Document Number: ICT-317669-METIS/D1.1). It is difficult to meet these requirements by means of single connectivity.
Moreover, the frequency range targeted in a non-backward compatible system in 5G, which may also be called NX (NeXt generation), covers 1 GHz up to 100 GHz. Due to addition of a high frequency spectrum to the cellular operation, high gain beamforming is a “must” to compensate negative effects due to unfavorable radio propagation properties. One issue with high gain beamforming is that a serving beam is optimal only for a small area, and expectedly a small fraction of time. When a user equipment (UE) moves, the serving beam may deteriorate very fast which makes UE mobility in NX a challenge.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 12, small cell enhancement has been studied. For higher layer enhancement, dual connectivity has been introduced to solve some problems, like power imbalance and mobility issues with densely deployment of small cells in heterogeneous networks. In 3GPP TR 36.842 V12.0.0, it is specified that a term “dual connectivity” is used to refer to an operation where a given UE consumes radio resources provided by at least two different network points connected with a non-ideal backhaul. Furthermore, each eNB involved in dual connectivity for a UE may assume different roles. Those roles do not necessarily depend on the eNB's power class and can vary among UEs. Dual connectivity consists in configuring a UE with one master eNB (MeNB) and at least one secondary eNB (SeNB). The UE can connect to one MeNB and one SeNB concurrently, while different frequencies are used between the MeNB and SeNB. The target of dual connectivity in LTE is to separate control plane from user plane and improve user throughput by aggregating transmissions from different nodes, but contributing less to system robustness or reliability and downlink/uplink decoupling.